3rd Generation Partnership Project (3GPP) as an asynchronous cellular mobile communication standard organization proceeds with the discussion to support wireless communication between mobile terminals or electronic devices, i.e., Device-to-Device (D2D) communication, as well as wireless communication between existing base stations and mobile terminals (user equipment), in Long Term Evolution (LTE) system specification.
One of the primary functions of 3GPP systems required for D2D communication is a function of supporting Public Safety services. That is, although a network is in an emergency state where it cannot provide normal services, e.g., a natural disaster, etc., 3GPP systems need to support LTE-based wireless communication between members in policeman group, firefighter group and/or government agent group, or between the groups. When an emergency occurs, it is more efficient to perform one-to-many communication in a particular group or the entire group, i.e., broadcast communication, rather than perform one-to-one communication between members. The 3GPP has agreed that broadcast communication may employ a D2D communication scheme in current Release-12 (Rel-12). Physical layer feedback of a closed-loop scheme, such as HARQ ACK/NACK, is likely not to be used, considering the features of one-to-many communication.
In D2D communication, radio resources used in a transmitting terminal may employ one of the following two methods. One method refers to a central resource allocation that allows a transmitting terminal to receive radio resources that it will use from a particular resource allocating entity. The particular resource allocating entity may serve as a base station in cellular communication. When networks do not normally provide services, a particular terminal may perform the resource allocation function. In this case, it is ideal to schedule radio resources of terminals in the coverage of the resource allocation terminal, thereby performing D2D communication without a conflict of radio resources, which may be advantageous.
However, the central resource allocation needs to additionally determine a method to select the particular resource allocating terminal and forces a terminal, supposed to perform resource allocation, to support functions, as of a base station. Therefore, the central resource allocation increases in the burden of complexity of terminal which is disadvantageous. The central resource allocation needs to define control channels for transmission/reception of the resource allocation information. When a number of terminals performing resource allocation are adjacent to each other, additional information is required to tune resource allocating terminals in order to prevent a conflict of resource allocation between terminals in the adjacent areas. Although the base station may perform transmission/reception of the tuning information via a wired network, it needs to define separate physical channels or signals for exchanging the turning information between the resource allocating terminals.
The other method refers to a distribution resource allocating method that allows a transmitting terminal to select radio resources that it will use for itself. The process of selecting radio resources in a transmitting terminal may be performed via the Channel Sense Multiple Access/Collision Avoidance (CSMA/CA). That is, a transmitting terminal performs a channel sensing process through a radio resource region set for D2D communication in order to determine whether a corresponding radio resource is currently used for D2D communication with other terminals. When the transmitting terminal ascertains that a corresponding radio resource is occupied by other terminals, it continues performing a channel sensing process to search for available radio resources, without using the radio resource. On the other hand, when the transmitting terminal ascertains that a corresponding radio resource is empty (not occupied), it may transmit its signals via the radio resource. The transmitting terminal in use of radio resources always needs to transmit a channel sensing signal for indicating that it is using the radio resources to other terminals performing a channel sensing process. A channel sensing signal is set to have a sequence-based signal structure, similar to a random access preamble, a Reference Signal (RS), etc.
The distribution resource allocating method using a channel sensing signal has a possibility that a conflict of resources may occur when a number of terminals that have performed a channel sensing process ascertain that a particular radio resource is empty and perform simultaneous transmission of their signals. On the other hand, the distribution resource allocating method does not need a resource allocating terminal performing operations, as of a base station. Therefore, the distribution resource allocating method does not have the burden of complexity of terminal, which is advantageous. The central resource allocating method does not always need the information regarding resource allocation, the information turning between resource allocating terminals, etc., described above, so that they can operate by performing, at least, a signaling, via a channel sensing signal. In order to resolve a conflict of resources between transmitting terminals, a random back-off may be employed. The random back-off refers to a procedure performed in such a way that: when terminals ascertain that a radio resource is empty via a channel sensing process, they respectively continue performing a channel sensing process for back-off times randomly selected. When the terminal does detect a channel sensing signal transmitted from other terminals and thus ascertains that a corresponding radio resource is empty, it starts transmission, or otherwise stops the back-off.
The distribution resource allocating method may also employ another method as follows. The method may include resource information that a transmitting terminal uses in transmitting data in a scheduling assignment (SA) signal and transmit it. Receiving terminals receive the SA signal, identify resources transmitting data of the transmitting terminal, and receive data via a corresponding resource. The resource region through which terminals are capable of transmitting/receiving SA signals may be preset. The settings of the resource region through which terminals are capable of transmitting/receiving SA signals may be known to individual terminals from a base station via system information or by upper layer signaling according to terminals. Alternatively, the range of resources may be set to be fixed and stored in memory devices of individual terminals.
The resource region capable of transmission/reception of SA may be set to have a time period and/or a frequency and may exist periodically, according to the settings. On time domain, the resource region capable of transmission/reception of SA may be followed by a resource region capable of transmitting/receiving data related to a resource region capable of transmission/reception of a corresponding SA. Each of the transmitting terminals is capable of selecting a resource to transmit it SA from the resource region capable of transmitting/receiving SA signals.
A first method of sharing data resources via scheduling assignment (SA) includes: implicitly relating SA resources of individual transmitting terminals to data resources according to a preset rule; decoding the SA signals in a receiving terminal; and informing the receiving terminal of a location of a data resource related to a corresponding SA from the location of a corresponding SA resource.
A second method of sharing data resources via scheduling assignment (SA) includes: explicitly informing a receiving terminal of a data resource related to SA signals of individual transmitting terminals; decoding the SA signals in the receiving terminal; and informing the receiving terminal of a location of a data resource related to a corresponding SA from the location of a corresponding SA resource.